Electrical resistivity has been used to evaluate characteristics of geological formations in drilling operations. As noted in the PRIOR ART depiction of FIG. 1, a downhole tool 200 is outfitted with one or more transmitter electrodes 202 and one or more imaging electrodes 204. The imaging electrodes 204 are often positioned in close proximity to the mud cake 206 of the uncased borehole 208. The transmitter electrode 202 then discharges an electrical current, a portion of which passes through the borehole fluid, mud cake 206, and geologic formation 210 and is received by the imaging electrode 204. The changes between the current received by the imaging electrode 204 and the current emitted by the transmitter electrode 202 can be interpreted to provide useful information about the porosity, density and other features of the geologic formation 210.
The use of borehole resistivity to directly evaluate the characteristics of the formation suffers from a significant drawback. Namely, a significant portion of current applied by the transmitting electrode 202 passes directly through the borehole 208 to the imaging electrode 204 (shown in dashed lines). Because the fluid in the borehole 208 may be significantly more conductive than the formation 210, the current tends to dissipate quickly through the borehole 208 and does not pass through the formation 210. The dissipation of current through the borehole 208 necessitates the use of higher powered transmitting currents, which is inefficient and yields a poor signal-to-noise ratio. Accordingly, it would be desirable to provide systems and methods that overcome the afore-described problems and drawbacks.